Lignin is the second most abundant natural polymer in the biosphere followed by cellulose, accounting for up to 30% of lignocellulosic biomass. Over 50 million tons of so-called technical lignin is annually extracted from lignocellulosic biomass as a byproduct of the pulp and paper industry with increasing amounts from the emerging cellulosic ethanol industry (Ragauskas et al., “Lignin Valorization: Improving Lignin Processing in the Biorefinery,” Science 344(6185):1246843 (2014) and Azadi et al., “Liquid Fuels, Hydrogen and Chemicals from Lignin: a Critical Review,” Renew. Sust. Energ. Rev. 21:506-523 (2013)). Lignin is a phenylpropane-based polymer biosynthesized from random polymerization of three precursor monomers. Because of its abundance and availability in low cost, lignin has potential as a renewable source of aromatics. At present, most lignin is burned as boiler fuel while only 2% of it is upgraded to biobased products, such as binder, resin, and dispersant (Huber et al., “Synthesis of Transportation Fuels from Biomass: Chemistry, Catalysts, and Engineering,” Chem. Rev. 106:4044-4098 (2006); Zakzeski et al., “The Catalystic Valorization of Lignin for the Production of Renewable Chemicals,” Chem. Rev. 110:3552-99 (2010); Effendi et al., “Production of Renewable Phenolic Resins by Thermochemical Conversion of Biomass: A Review,” Renew. Sust. Energ. Rev. 12:2092-2116 (2008); and Lange et al., “Oxidative Upgrade of Lignin—Recent Routes Reviewed,” Eur. Polym. J. 49:1151-1173 (2013)). Developing effective upgrading methods to convert lignin into value-added products could reduce dependency on petroleum and also improve the economic prospects for companies producing lignin as a co-product of biomass processing.
Depolymerizing lignin by either biological or thermochemical means has been extensively studied over the past several decades (Perez et al., “Biodegradation and Biological Treatments of Cellulose, Hemicellulose and Lignin: An Overview,” Int. Microbiol. 5:53-63 (2002); Salvachua et al., “Towards Lignin Consolidated Bioprocessing: Simultaneous Lignin Depolymerization and Product Generation by Bacteria,” Green Chem. 17:4951-4967 (2015); Pandey et al., “Lignin Depolymerization and Conversion: a Review of Thermochemical Methods,” Chem. Eng. Technol. 34:29-41 (2011); and Amen-Chen et al., “Production of Monomeric Phenols by Thermochemical Conversion of Biomass: A Review,” Bioresource Technol. 79:277-299 (2001)). Solvent liquefaction using a variety of solvents at elevated temperatures and pressures is the most common method for deconstructing lignin (Jin et al., “Liquefaction of Lignin by Polyethyleneglycol and Glycerol,” Bioresource Technol. 102:3581-3583 (2011); Kang et al., “Hydrothermal Conversion of Lignin: A Review,” Renew. Sust. Energ. Rev. 27:546-558 (2013) and Kleinert et al., “Optimizing Solvolysis Conditions for Integrated Depolymerisation and Hydrodeoxygenation of Lignin to Produce Liquid Biofuel,” J. Anal., Appl. Pyrol. 85:108-117 (2009)). Despite the advantages of solvent liquefaction, the consumption of solvent and the need for separating reaction products from solvent, char and/or catalysts present both economic and technical challenges.
Fast pyrolysis is an alternative thermal depolymerization technique. Fast pyrolysis has been widely explored for the conversion of whole plant biomass into liquid products (Bridgwater et al., “An Overview of Fast Pyrolysis of Biomass,” Org. Geochem. 30:1479-1493 (1999); Mohan et al., “Pyrolysis of Wood/Biomass for Bio-Oil: A Critical Review,” Energ. Fuel 20:848-889 (2006) and Bridgwater “Upgrading Biomass Fast Pyrolysis Liquids,” Environ. Prog. Sust. Energ. 31:261-268 (2012)). Fast pyrolysis rapidly heats the feedstock (usually <2 s) in the absence of oxygen and usually at atmospheric pressure. Lignocellulosic biomass is substantially decomposed to form liquid, called bio-oil, char, and non-condensable gases. Bio-oil from fast pyrolysis is similar to the liquid produced by solvent liquefaction. It can be upgraded to hydrocarbon fuels or other chemicals. The char product (also known as biochar) has several applications such as solid fuel, soil amendment and activated carbon (Laird et al., “Review of the Pyrolysis Platform for Coproducing Bio-Oil and Biochar,” Biofuel Bioprod Bior. 3:547-562 (2009)).
Despite its potential, fast pyrolysis of technical lignin has been infrequently studied, usually limited to batch experiments involving only small quantities of lignin (Ben et al., “Pyrolysis of Kraft Ligning with Additives,” Energ. Fuel 25:4662-4668 (2011); Kosa et al., “Pyrolysis Oils from CO2 Precipitated Kraft Lignin,” Green Chem. 13:3196-3202 (2011) and Sharma et al., “Characterization of Chars From Pyrolysis of Lignin,” Fuel 83:1469-1482 (2004)). Although these previous studies have provided valuable insight into lignin depolymerization, continuous pyrolysis relevant to commercial applications are lacking (De Wild et al., “Lignin Pyrolysis for Profitable Lignocellulosic Biorefineries,” Biofuel Bioprod. Bior. 8:645-657 (2014)). Efforts to continuously pyrolyze lignin have been largely unsuccessful due to the melting and subsequent agglomeration of lignin particles to form “hard shell” solid material, which clogs the reactor and forces shut-down. This problem was highlighted by an international collaboration in 2010 (Nowakowski et al., “Lignin Fast Pyrolysis: Results From an International Collaboration,” J. Anal., Appl. Pyrol. 88:53-72 (2010)). Two types of lignin, one from soda pulping of non-woody biomass and the other from weak acid hydrolysis of softwood, were distributed to seven laboratories for pyrolysis in small-scale, fluidized bed pyrolyzer. None of the laboratories were able to pyrolyze the soda pulp lignin due to plugging of the feeder or defluidization of the reactor. Pyrolysis of the acid hydrolysis lignin was only marginally better, possibly because of the presence of a large amount of carbohydrate in this particular feedstock.
Several researchers have attempted to pyrolyze lignin by adding cooling jackets or making other design changes to the feeder tubes, installing mechanical stirrers inside the reactors, pelletizing lignin, or performing pyrolysis in the presence of oxygen (Li et al., “Oxidative Pyrolysis of Kraft Lignin in a Bubbling Fluidized Bed Reactor With Air,” Biomass Bioenerg. 76:96-107 (2015); Trinh et al., “Fast Pyrolysis of Lignin Using a Pyrolysis Centrifuge Reactor,” Energ. Fuel 27:3802-3810 (2013); and Tumbalam Gooty et al., “Kraft-Lignin Pyrolysis and Fractional Condensation of its Bio-Oil Vapors,” J. Anal., Pyrol. 106:33-40 (2014)). None of these efforts were completely successful in eliminating agglomeration. In a recent review paper, Ragauskas et al., “Lignin Valorization: Improving Lignin Processing in the Biorefinery,” Science 344(6185):1246843 (2014) warned that the inability to continuously feed lignin to large-scale reactors is the primary technical barrier to pyrolyzing lignin for the production of fuels and chemicals.
Pretreating lignin is another approach to improving the pyrolysis of lignin. Mukkamala et al., “Formate-assisted Fast Pyrolysis of Lignin,” Energ. Fuel 26:1380-1384 (2012) pretreated an acid hydrolyzed kraft lignin with 50-100% weight equivalence of calcium formate using a two-step process before pyrolysis in an entrained flow reactor. In their study, fluidizing sand was removed from the reactor during pyrolysis in order to avoid agglomeration between sand and lignin particles. WO 2011159154 A1 to Wilberink et al. describes a slurry of lignin and clay (1:1 weight ratio) mixed in water and extruded as pellets and then dried in a two-step heat treatment prior to pyrolysis. Upon pyrolysis in an auger reactor, phenolic-rich bio-oil was produced. The solid residue from pyrolysis of lignin-clay pellets were pellets containing char and the clay binder. See Wilberink et al. De Wild et al., “Lignin Pyrolysis for Profitable Lignocellulosic Biorefineries,” Biofuel Bioprod. Bior. 8:645-657 (2014) also pyrolyzed in a fluidized bed the pellets formed from the lignin co-product of organosolv processing of straw. They concluded that a dedicated feeding system and careful control of pyrolysis conditions are required to pyrolyze lignin even when pelletized.
Because of this difficulty of pyrolyzing lignin, the products of lignin pyrolysis have yet to be fully characterized. One previous study reported the composition of pyrolysis vapor but not char (De Wild et al., “Lignin Pyrolysis for Profitable Lignocellulosic Biorefineries,” Biofuel Bioprod. Bior. 8:645-657 (2014)). A study conducted by Sharma et al., “Characterization of Chars From Pyrolysis of Lignin,” Fuel 83:1469-1482 (2004) characterized the agglomerated char produced from slow pyrolysis of lignin in a batch reactor but did not give information about volatile products.
The present invention is directed to overcoming the deficiencies in the art by treating lignin to prevent its melting and subsequence agglomeration upon heating, and preventing difficulties that can arise from lignin agglomeration during pyrolysis, and allowing for continuous pyrolysis in a fluidized reactor.